Speech recognizing circuit



DeC- 23, 1939 D. F. HooPER, JR

SPEECH RECOGNIZNG CIRCUIT Filed March 6, 196'? www. u@

INVENTOR. www A/mfe, Je

United safes Pat-em o" 3,485,951 SPEECH RECOGNIZING CIRCUIT Donald F.Hooper, Jr., 6803 Lotus Ave., San Gabriel, Calif. 91775 Filed Mar. 6,1967, Ser. No. 620,699 Int. Cl. H03h 7/ 46 U.S. Cl. 179--1 11 ClaimsABSTRACT OF THE DISCLOSURE The formant frequencies characteristic of avowel are automatically modified to compensate for changes in pitch ofthe speaker by biasing an audiofrequency amplifier so as to distort theelectrical signal representing the vowel sound. A tank circuitconfiguration for indicating the presence of a particular vowel soundcomprises a plurality of L-C parallel circuits, one tuned for eachformant frequency. The inductors of all the parallel circuits areinductively coupled to an inductor connected across the output of theaudiofrequency amplifier. The inductors are toroidal coils stacked inalternating fashion one on top of the other.

BACKGROUND OF THE INVENTION This invention relates to electricalcircuits and, more particularly, to a circuit for recognizing andindicatlng the presence of human sounds.

'Ille mode of operation of one class of speech recognizing circuits isbased on the phenomenon that the pure vowel sounds are each uniquelycharacterized by certain combinations of frequency components. Thus,each vowel sound includes at least three frequency components, calledformants in the art, which are not found in the same combination in anyof the other pure vowel sounds. The English lan-guage contains at leastten such pure vowel sounds, each of which is uniquely characterized byits three formant frequencies. To recognize and indicate the presence ofa particular vowel in one prior art arrangement, the sound energy isconverted to an electrical signal that is applied to three tank circuitstuned to the formant frequencies characteristic of the vowel. The tankcircuits are connected to the input of an AND gate. When the vowel soundis spoken, all the tank circuits resonate, with the result that the ANDgate produces an output.

Although each vowel sound is uniquely characterized by three formantfrequencies regardles of the pitch of the speaker, the particular valuesof these formant frequencies are dependent upon the pitch of thespeaker. Accordingly, to accommodate male, female, and juvenilespeakers, speech recognizing circuits based on the formant phenomenonmust be adjustable. If the speech recognizing circuit is set to operateupon a low-pitched speaker and it is desired to change to a high-pitchedspeaker, the tank circuits associated with each vowel must be returnedso they resonate at the new, higher formant frequencies.

SUMMARY OF THE INVENTION According to the invention, the formantfrequencies characteristic of a vowel are automatically modified tocompensate for changes in pitch of the speaker, so regardless of thespeakers pitch, no adjustment of the tank circuits that detect theformant frequencies is necessary. It has been discovered thatcompensation for pitch occurs in an audiofrequency amplifier when it isbiased to distort the electrical signal representing the vowel sound. Asa result, a mixing action takes place and modulation components aregenerated that remain sub- Patented Dec. 23, 1969 stantially constantfor a particular vowel sound regardless of the pitch of the speaker.Consequently, each tank circuit must only be tuned to a single frequencyvalue. Preferably, the tank circuits are tuned to accommodate. anaverage male speaker.

A feature of the invention is an advantageous tank circuit configurationto indicate the presence of a particular vowel sound. As a tank circuitfor each formant frequency, an L-C parallel circuit is provided. Theinductors of all parallel circuits are inductively coupled to aninductor connected across the output of the audiofrequency amplifier. Ifany of the tank circuits is not resonating, it loads the output of theamplifier so a small signal is inductively coupled to each of the tankcircuits. Only in the case where all of the tank circuits are resonatingis the output of the audiofrequency amplifier not loaded. Then, a largesignal appears across the tank circuits to indicate the simultaneouspresence of the three formant frequencies characteristic of the vowelsound. Most advantageously, the inductors are toroidal coils stacked oneon top of the other within a shielded housing. The inductor connectedacross the output of the audiofrequency amplifier comprisesseries-connected coils positioned between each tank circuit coil in thestack.

BRIEF DESCRIPTION OF THE DRAWING The features of the invention areillustrated in the drawing of a specific embodiment, in which:

FIG. l is a schematic circuit diagram of a speech recognizing circuitembodying the principles of the invention;

FIG. 2 is a block diagram of a modification of the speech recognizingcircuit of FIG. l; and

FIG. 3 is a schematic diagram representing the physical relationshipbetween the coils employed to detect formant frequencies.

DESCRIPTION OF THE SPECIFIC EMBODIMENT In FIG. l, a microphone 1 isconnected to the input of an audiofrequency amplifier having stages 2,3, 4, and 5 connected in tandem. Stages 2, 3, and 4, which are allidentical, each comprise a transistor connected in the common emitterconfiguration. In the interest of simplicity, only stage 2 is providedwith reference numerals, it being understood that stages 3 and 4 eachhave the same circuit components. The base of a transistor 6 constitutesthe input of stage 2 to which microphone 1 is coupled. A voltage dividercomprising resistors 7 and 8 is connected between a source 9 of negativepotential and ground. The base of transistor 6 is tied to the junctionof resistors 7 and 8, so the ratio of resistance thereof determines theapplied base bias. A load-resistor 10 is connected between source 9 andthe collector of transistor 6, while a coupling capacitor 11 isconnected between the collector of transistor 6 and the input to stage3. A resistor 12 and a capacitor 13 are connected in parallel betweenthe emitter of transistor 6 and ground.

Stage 5 comprises a transistor 16 connected in the com mon collectorconfiguration. The base of transistor 16 forms the input of stage 5 towhich the output of stage 4 is coupled. The base of transistor 16 isalso connected by a resistor 17 to source 9. A resistor 18 and acapacitor 19 are connected in parallel between source 9 and thecollector of transistor 16. The emitter of transistor 16, which iscoupled to ground by a resistor 20, is connected to the movable arm of aselector switch 25.

The stages of the audiofrequency amplifier are biased so the electricalsignals representing the vowel sounds are distorted while passingthrough the amplifier. Preferably, linear amplification takes place instages 2, 3, and 4, while the signal is greatly distorted in stage 5,preferably so both peaks of the signal are substantially clipped. It hasbeen discovered that the distortion results in the generation ofmodulation components that remain substantially constant for anyparticular vowel sound irrespective of the pitch of the speakers voice.Typical component values for the circuit are as follows: microphone 1 is500 ohms; resistor 7 is 100,000 ohms; resistor 8 is 10,000 ohms;resistor 10 is 4,700 ohms; resistor 12 is 1,000 ohms; capacitor 11 is 5microfarads; capacitor 13 is 10 microfarads; resistor 17 is 1 millionohms; resistor 18 is 4,700 ohms; resistor 20 is 39,000 ohms; capacitor19 is 10 microfarads; source 9 is 18 volts; and transistors 6 and 16 areType 2Nl414.

The signal developed at the output of the audiofrequency amplifier isdirected to one of a plurality of filters 21, 22, 23, and 24 by means ofselector switch 25. Filters 21 through 24 each correspond to a differentvowel sound and are identical except for the resonant frequencies towhich they are tuned. Filter 24 is shown in detail. It comprises aninductor 30 and an inductor 31 connected in series between one contactof selector switch 25 and ground. An inductor 32, which is connected inparallel with a capacitor 33 to form a tank circuit, is inductivelycoupled to inductor 30; an inductor 34, which is connected in parallelwith a capacitor 35 to form a second tank circuit, is inductivelycoupled to inductor 31; and an inductor 36, which is connected inparallel with a capacitor 37 to form a third tank circuit, isinductively coupled to inductors 30 and 31. Most advantageously,inductors 30, 31, 32, 34, and 36 are toroidal or so-called Rowland coilsphysically arranged in axial alignment with one another in a stack, asrepresented in FIG. 3. The inductors are electrically isolated from, butmagnetically coupled to, one another. In addition, al1 the circuitcomponents of the lter are enclosed in a magnetically shielded housing.Toroidal coils have a high Q value, i.e., a large inductance toresistance ratio. Therefore, very selective filtering of the formantfrequencies is possible. Generally, it is difficult, however, to coupleenergy magnetically between toroidal coils. The physical arrangement oftoroidal coils shown in FIG. 3 provides exceptionally good magneticcoupling. Each of the three tank circuits of filter 24 is tuned to adifferent formant frequency of the vowel associated with filter 24.Preferably, the tank circuits are tuned to the formant frequency valuesfor a typical male voice. If one or more of the tank circuits is notresonating, filter 24 loads the output of the audiofrequency amplifier.This is indicated by a low signal level at an output terminal 38 offilter 24, which is connected to one of the tank circuits (arbitrarilytaken as the tank circuit comprising inductor 36 and capacitor 37). Onlywhen all of the tank circuits are resonating does filter 24 fail to loadthe output of the audiofrequency amplifier. At this time, a high signallevel appears at output terminal 38.

To indicate the presence of a different vowel sound, selector switch 2Sis moved to connect the filter corresponding to that vowel sound to theoutput of the audiofrequency amplifier. The arrangement of FIG. 1 inwhich the filters are coupled to the audiofrequency amplifier through aselector switch is particularly well adapted to use in teaching deafpersons to speak. In such case, selector switch 25 is placed in theposition corresponding to the vowel sound to be practiced by the deafperson who speaks into microphone 1. When the vowel sound is correctlyuttered, a high signal level appears at the output terminal of thefilter. This condition could be observed by the deaf person by reading avoltmeter connected to the output of the lter.

There are, of course, many other applications for the speech recognizingcircuit of the invention. For example, it could be employed to actuatean adding machine verbally by assigning a number to each vowel sound. Insuch case, the arrangement of FIG. 2 might be employed. The speakertalks into a microphone 42 which is conuccted to the input ot' anaudiofrequency amplifier 43.

Audiofrequency amplifier 43 is identical to the amplifier shown in FIG.1 except it has a plurality of common collector stages (corresponding tostage 5 in FIG. l), each connected to the output of the preceding stage(corresponding to stage 4 in FIG. 1). One common collector stage isprovided for each vowel sound to be recognized. A filter (one of filters44 through 47) is connected to the output of each common collector stageto produce an indication upon the appearance of corresponding vowelsounds. Thus, the filters are isolated from one another by the commoncollector stages. Filters 44 through 47 could be constructed like filter24. The outputs of filters 44 through 47 are all coupled to autilization device 48, which could be an adding machine.

What is claimed is:

1. A speech recognizing cirucit comprising: a source of audiofrequencysignals; an audiofrequency amplifier to which the source is coupled, theamplifier being biased to distort the audiofrequency signal in thecourse of amplification; and means coupled to the output of theaudiofrequency amplifier for producing an indication upon thesimultaneous appearance of the formant frequencies of a vowel.

2. The circuit of claim 1, in which the source is a microphone.

3. The circuit of claim 1, in which the means for producing anindication is a plurality of tank circuits, one tuned to each formantfrequency, the tank circuits being arranged to produce an output onlywhen they are all resonating.

4. The circuit of claim 1, in which the means for producing anindication comprises: a plurality of inductors connected in seriesacross the output of the audiofrequency amplifier; a plurality ofparallel resonant circuits, each tuned to a formant frequency of avowel, each resonant circuit including an inductor and a capacitor; eachof the series-connected inductors being inductively coupled to two ofthe resonant circuits so that any of the resonant circuits notresonating loads the output of the amplifier.

5. The circuit of claim 1, in which the means for producing anindication comprises: an inductor connected across the output of theaudiofrequency amplifier; a plurality of tank circuits, each includingan inductor and a capacitor in parallel; and means for coupling each ofthe tank circuits to the inductor so that any of the tank circuits notresonating loads the output of the amplifier.

6. The circuit of claim 1, in which the means for producing anindication is a plurality of tank circuits, one tuned to each formantfrequency of an average male speaker, the tank circuits being arrangedto produce an output only when they are all resonating.

7. In a speech recognizing circuit the combination comprising: a sourceof electrical signals representing vowel sounds; an inductor connectedacross the source; a plurality of tank circuits each tuned to adifferent formant frequency of a vowel, each tank circuit including theparallel combination of an inductor and a capacitor; and means forcoupling the inductor of each of the tank circuits to the inductorconnected across the source so that any of the tank circuits notresonating loads the source.

8. The combination of claim 7, in which: the inductor connected acrossthe source is divided into a first part and a second part; the pluralityof tank circuits includes a first tank circuit, a second tank circuit,and a third tank circuit; the first part of the inductor connectedacross the source is inductively coupled to the inductors of the firstand second tank circuits; and the second part of the inductor connectedacross the source is inductively coupled to the inductors of the secondand third tank circuits.

9. The combination of claim 8, in which the inductors of the tankcircuits and the first and second parts of the inductor connected acrossthe source are each toroidal coils arranged in axial alignment with oneanother in a stack with the first part lying between the inductors ofthe rst and second tank circuits and the second part lying between theinductors of the second and third tank circuits.

10. The combination of claim 7, in which the source is an amplier thatdistorts electrical signals representing vowel sounds.

11. A speech recognizing circuit comprising: a source of audiofrequencysignals including a given vowel sound; tank circuit means tuned to theformant frequencies of the given vowel sound produced by a speaker witha particular pitch, the tank circuit means generating an indication uponthe simultaneous application thereto of all the formant frequencies towhich the tank circuit means are tuned; means for producing modulationcomponents from the audiofrequency signal that remain substantiallyconstant at the formant frequencies to which the tank circuit means aretuned as the pitch of the speaker varies;

3,188,566 6/1965 Bullene.

OTHER REFERENCES W. C. Dresch: Improved Vowel Separation for Speech mRecognition Applications; IBM Technical Disclosure Bulletin; vol. 5, No.5, October, 1962.

KATHLEEN H. CLAFFY, Primary Examiner C. HRA-UGH, Assistant Examiner U.S.C1. XR. 324-77

